1. Field of the Invention
The present invention relates to methods and apparatus of producing and polishing a semiconductor device, more particularly relates to methods of producing and polishing a semiconductor device including a step of reducing surface unevenness accompanying the formation of a metal film, and to a polishing apparatus thereof.
2. Description of the Related Art
Along with the increase in integration and reduction of size of semiconductor devices, progress has been made in miniaturization of interconnections, reduction of interconnection pitch, and superposition of interconnections. The importance of the multilayer interconnection technology in the manufacturing process of semiconductor devices is therefore rising.
Aluminum has been frequently used as an interconnection material of a semiconductor device having a multilayer interconnection structure, but in order to reduce the propagation delay of signals in the recent 0.25 xcexcm or less design rule, has been attempted active development of an interconnection process in that aluminum as the interconnection material is replaced by copper. When using copper for interconnections, it is beneficial that both a low resistance and a high electromigration tolerance can be obtained.
In a process using this copper for interconnections, for example, an interconnection process referred to as the damascene process for burying a metal in a groove-like interconnection pattern formed in an interlayer insulation film in advance and removing excess metal film by chemical mechanical (mechno-chemical) polishing (CMP) to form the interconnections has become influential. The damascene process has the features that etching of the interconnections become unnecessary and also a further upper interlayer insulation film becomes flat by itself, so the manufacturing steps can be simplified.
Further, by the dual damascene process, where not only grooves for the interconnections, but also the contact holes are formed as grooves in the interlayer insulation film and where the interconnections and the contact holes are simultaneously buried by the metal, a greater reduction of the interconnection steps can be achieved.
Here, an explanation will be made of an example of the process for forming copper interconnections by the dual damascene process with reference to the accompanying drawings.
First, as shown in FIG. 25A, for example, an interlayer insulation film 302 made of silicon oxide is formed by low pressure chemical vapor deposition (LP-CVD) on a silicon or other semiconductor substrate 301 on which a not illustrated impurity diffusion region is appropriately formed.
Next, as shown in FIG. 25B, contact holes CH communicating with the impurity diffusion region of the semiconductor substrate 301 and grooves M in which it will be formed a predetermined pattern of interconnections to be electrically connected to the impurity diffusion region of the substrate 301 which are formed by using well-known photolithography and etching.
Next, as shown in FIG. 25C, a barrier film 305 is formed on the surface of the interlayer insulation film 302 and in the contact holes CH and the grooves M. This barrier film 305 is formed by a material such as Ta, Ti, TaN, or TiN by well-known sputtering. When the interconnection material is copper and the interlayer insulation film 302 is silicon oxide, since copper has a large diffusion coefficient with respect to silicon oxide, it is easily oxidized. The barrier film 305 is provided to prevent this.
Next, as shown in FIG. 26A a seed copper film 306 is formed on the barrier film 305 to a predetermined thickness by well-known sputterings.
Then, as shown in FIG. 26B a copper film 307 is formed so as to bury the contact holes CH and the grooves M by copper. The copper film 307 is formed by the process of plating, CVD, sputtering, etc.
Next, as shown in FIG. 26C the excess copper film 307 and barrier film 305 on the interlayer insulation film 302 are removed by CMP for flattening.
Due to the above steps, copper interconnections 308 and contacts 309 are formed.
By repeating the above process on the interconnections 308, multilayer interconnections can be formed.
Summarizing the disadvantages to be solved by the invention, in the step of removing the excess copper film 307 by CMP in the copper interconnection forming process using the dual damascene process, because the flattening technique employing conventional CMP involves applying a predetermined pressure between a polishing tool and the copper film for polishing, it suffers from a large damage given to the semiconductor substrate. Especially in a case where an organic insulation film of a small dielectric constant having a low mechanical strength is adopted for the interlayer insulation film, this damage no longer becomes negligible and may cause cracks of the interlayer insulation film and separation of the interlayer insulation film from the semiconductor substrate.
Further, the removal performance differs among the interlayer insulation film 302, the copper film 307, and the barrier film 305, therefore it suffers from the disadvantage that dishing, erosion (thinning), recesses, etc. easily occur in the interconnections 308.
Dishing is a phenomenon where, as shown in FIG. 26, when there is an interconnection 308 having a width of for example about 100 xcexcm at for example a 0.18 xcexcm design rule, the center portion of the interconnection is excessively removed and sinks. If this dishing occurs, the sectional area of the interconnection 308 becomes insufficient. This causes poor interconnection resistance etc. This dishing is apt to occur when copper or aluminum, which is relatively soft, is used as the interconnection material.
Erosion is a phenomenon where, as shown in FIG. 27, a portion having a high pattern density such as where interconnections with a width of 1.0 xcexcm are formed at a density of 50% in a range of for example 3000 xcexcm is excessively removed. When erosion occurs, the sectional area of the interconnections becomes insufficient. This causes poor interconnection resistance etc.
Recess is a phenomenon where, as shown in FIG. 28, the interconnection 308 becomes lower in level at the interface between the interlayer insulation film 302 and the interconnection 308 resulting in a step difference. In this case as well, the sectional area of the interconnection becomes insufficient, causing poor interconnection resistance etc.
Further, in the step of flattening and removing the excess copper film 307 by CMP, it is necessary to efficiently remove the copper film. The amount removed per unit time, that is, the polishing rate, is required to be for example more than 500 nm/min.
In order to obtain this polishing rate, it is necessary to increase the polishing pressure on the wafer. When the polishing pressure is raised, as shown in FIG. 29, a scratch SC and chemical damage CD are apt to occur in the interconnection surface. In particular, they easily occur in the soft copper. For this reason, it causes opening of the interconnections, short-circuiting, poor interconnection resistance, and other defects. Further, if the polishing pressure is raised, there is the inconvenience that the amount of the scratches, separation of interlayer insulation film, dishing, erosion, and recesses also becomes larger.
A first object of the present invention is to provide a method of producing a semiconductor device capable of easily flattening an initial unevenness, excellent in efficiency of removal of an excess metal film, and capable of suppressing damage to an interlayer insulation film below a metal film when flattening the metal film by polishing; a second object of the present invention is to provide a method of polishing the same semiconductor device; a third object of the present invention is to provide a polishing apparatus using these methods.
An object of the present invention is to provide a method of production and a method of polishing a semiconductor device capable of easily flattening an initial unevenness, excellent in efficiency of removal of an excess metal film, and capable of suppressing damage to an interlayer insulation film below a metal film when flattening the metal film by polishing and to a polishing apparatus used for these methods.
To attain the above object, according to a first aspect of the present invention, there is provided a method of producing a semiconductor device, including the steps of forming an interconnection groove in an insulation film formed on a substrate, stacking a copper film having unevenness on its surface corresponding to the step difference of the interconnection groove on the entire surface of the insulation film so as to bury the interconnection groove, interposing an electrolytic solution including a chelating agent between a cathode member and the copper film, applying a voltage between the cathode member functioning as a cathode and the copper film functioning as an anode to oxidize the surface of the copper film and form a chelate film of oxidized copper, selectively removing a projecting portion of the chelate film corresponding to unevenness of the copper film to expose the copper film of that projecting portion at its surface, and repeating the chelate film forming step and the above chelate film removing step until the projecting portion of the copper film is flattened.
According to the above method for producing a semiconductor device, the uneven copper film surface formed when burying an interconnection groove by copper film is oxidized by anodic oxidation. This oxidized copper is chelated by a chelating agent in an electrolytic solution. Therefore, a chelate film of rather low mechanical strength able to be easily removed is formed. If removing a projecting portion of the chelate film, because the further exposed copper is chelated after being oxidized by anodic oxidation, flatness of the copper film is achievable by repeating the step of removing the projecting portion of the chelate film.
Since the resistance of the chelate film is higher than copper, the copper covered by the not removed chelate film remaining in the groove is hard to be oxidized by anodic oxidation by passing a current, so the chelation is very slow and the chelate film is formed by anodic oxidation only at the projecting portion of the copper exposed by the removal of the previous chelate film.
Further, because the current is supplied through an electrolytic solution, if the potential difference between the copper film on the anode and the cathode member of the cathode is constant, the current density becomes larger the shorter the distance between electrodes. Therefore, in the copper film exposed after removing the chelate film, the more projecting a part of the copper film is, the shorter the electrode distance to the cathode member used as the cathode and thus the higher the current density and consequently, the higher the speed of the anodic oxidation and the faster the chelation.
Accordingly, because of the accelerated chelation of the projecting portion of the copper film, it is able to achieve efficient flattening and suppression of damage to the interlayer insulation film below a copper film.
The method for producing a semiconductor device according to the present invention preferably further includes a step, after flattening the projecting portion of the copper film, of removing the chelate film formed on the surface of the copper film until removing the copper film stacked outside the interconnection groove.
Due to this, copper interconnections can be formed while suppressing damage to the interlayer insulation film below the copper film.
In the method for producing a semiconductor device according to the present invention, preferably, in the step of applying a voltage by using the cathode member as a cathode, a voltage is applied using as a cathode a conductive polishing tool for removing a projecting portion of the chelate film.
By using a polishing tool as the cathode, efficient chelation due to the anodic oxidation and efficient removal of the chelate film can be obtained.
In the method for producing a semiconductor device according to the present invention, preferably, in the step of applying a voltage by using the copper film as the anode, a voltage is applied on an anode member contacting or close to the copper film, making the copper film an anode through the electrolytic solution.
By locally passing a current from the anode member to the copper film through the electrolytic solution, a stable current can be supplied.
In this case, a current is supplied to the copper film from the anode member via the electrolytic solution and, further, from the copper film to the cathode member through the electrolytic solution, so the copper film near the cathode member is oxidized and chelated.
In the method for producing a semiconductor device according to the present invention, preferably, in the step of applying a voltage by using the cathode member as a cathode, a voltage is applied using a conductive electrode plate arranged parallel with the copper film as a cathode.
By arranging the electrode plate parallel with the copper film, as shown above, in the exposed copper films, the more projecting the portion, the shorter the electrode distance. Due to the increase of the speed of the anodic oxidation caused by the increased current density, the chelation is accelerated, so efficient flattening is achievable.
In the method for producing a semiconductor device according to the present invention, preferably, in the step of removing the chelate film, the chelate film is removed by wiping or mechanical polishing.
Since the chelate film has a rather low mechanical strength, mechanical polishing involving strong pressing is not necessary. It can be easily removed by wiping or mechanical polishing involving only weak pressing.
In the method for producing a semiconductor device according to the present invention, preferably, in the step of removing the chelate film, the chelate film is removed by applying vibration to the substrate. Alternatively, in the step of removing the chelate film, the chelate film is removed by flushing with the electrolytic solution.
Since the chelate film has a rather low mechanical strength, it can be easily removed not only by mechanical polishing, but also by vibration and the flushing action of the electrolytic solution.
In the method for producing a semiconductor device according to the present invention, preferably, in the chelate film forming step and the chelate film removing step, a current flowing through the cathode member and the copper film is monitored and the polishing process of the copper film is controlled in response to the magnitude of the current.
For example, by using a chelating agent forming a chelate film having a higher electrical resistance than the copper film, before the projecting portion is flattened, the current between the cathode member and the copper film increases when the projecting chelate film is removed because copper is exposed. When a chelate film is formed on the exposed copper again, the value of the current decreases. This cycle is repeated. When the copper film is flattened, because the chelate film on the copper film is removed completely and the copper film is exposed totally, the current reaches a maximum first, then the current value exhibits a new maximum at each removal.
When the barrier film is exposed, since usually the resistance of the barrier film is higher than copper, the current value begins to decrease after the chelate film is removed. Therefore, if stopping the application of voltage at the time when the current value begins to decrease, the formation of the chelate film by the anodic oxidation can be stopped after that time, thus the progress of the polishing can be controlled.
In addition, to achieve the above object, according to a second aspect of the present invention, there is provided a polishing method for polishing an object having a copper film on the surface to be polished, including the steps of interposing an electrolytic solution including a chelating agent between a cathode member and the polished surface, applying a voltage between the cathode member functioning as a cathode and the polished surface functioning as an anode to oxidize the surface of the copper film and form a chelate film of an oxidized copper film, selectively removing a projecting portion of the chelate film corresponding to the shape of the copper film to expose the copper film of that projecting portion at its surface, and repeating the above chelate film forming step and the chelate film removing step until the projecting portion of the copper film is flattened.
According to the polishing method of the present invention for polishing an object having a copper film on the surface to be polished, by interposing an electrolytic solution including a chelating agent between a cathode member and the polished surface and applying a voltage between the cathode member used as a cathode and the polished surface of the polished object used as an anode, the uneven copper surface is oxidized by the anodic oxidation. This oxidized copper is chelated by the chelating agent in the electrolytic solution, forming a chelate film of rather low mechanical strength thus able to be removed easily. If selectively removing a projecting portion of the chelate film, because the copper further exposed thereby is chelated after being oxidized by the anodic oxidation, flatness of the copper film is achievable by repeating the step of selectively removing the projecting portion of the chelate film.
Since the resistance of the chelate film is higher than copper, the copper covered by the unmoved chelate film remaining in the groove is hard to be oxidized by the anodic oxidation by supplying a current, so the chelation is very slow and a chelate film is formed by the anodic oxidation only at the projecting portion of copper exposed by removing the chelate film.
Further, because the current is supplied through an electrolytic solution, if the potential difference between the copper film on the anode and the cathode member of the cathode is constant, the current density becomes larger the shorter the distance between electrodes. Therefore, in the copper film exposed after removing the chelate film, the more projecting a part of the copper film is, the shorter the electrode distance to the cathode member used as the cathode and thus the higher the current density and consequently, the higher the speed of the anodic oxidation and the faster the chelation.
Accordingly, because of the accelerated chelation of the projecting portion of the copper film, it is possible to achieve efficient flattening and suppression of damage to an interlayer insulation film below a copper film.
In addition, to achieve the above object, according to a third aspect of the present invention, there is provided a method for production of a semiconductor device, including the steps of forming at least a groove or hole in an insulation film formed on a substrate, stacking a metal film on the insulation film so as to bury the groove or hole, interposing an electrolytic solution between a cathode member and the metal film, applying a predetermined voltage between the cathode member used as a cathode and the metal film used as an anode, removing the surface of the metal film, and repeating the above step of removing the metal film until the unevenness of the surface of the metal film is reduced.
Further, the method for producing a semiconductor device of the present invention further includes a step of forming a barrier film for preventing diffusion of the metal film to the insulation film on the insulation film so as to bury the groove or hole after forming the groove or hole in the insulation film and before stacking the metal film on the insulation film, wherein the metal film is stacked on the barrier film in the step of stacking the metal film on the insulation film.
Further, in the step of removing the surface of the metal film, the step of copper film removal is repeated until the metal film stacked outside the groove or hole is removed.
According to the above method for producing a semiconductor device, when processing the surface of a metal film buried in a groove or hole, by interposing an electrolytic solution between a cathode member and the metal film and applying a predetermined voltage between the cathode member used as a cathode and the metal film used as an anode, the metal film is oxidized by the anodic oxidation, ionized in a state of metallic ions, and has a very low mechanical strength enabling it to be easily removed. If removing the oxidized metal film, the further exposed metal film is oxidized by the anodic oxidation again. By repeating the step of removing the metal film after the anodic oxidation, the step difference of the metal film can be reduced.
Further, because current is supplied through an electrolytic solution, if the potential difference between the metal film of the anode and the cathode member of the cathode is constant, the current density becomes larger the shorter the distance between electrodes. Therefore, in the copper film exposed after removing the chelate film, the more projecting the copper film is, the shorter the electrode distance to the cathode member used as the cathode and thus the higher the current density. Consequently, the anodic oxidation of the projecting portion of the metal film is accelerated, the step difference of the metal film surface can be efficiently reduced, and the damage to the insulation film below the metal film can be suppressed.
In addition, to achieve the above object, according to a fourth aspect of the present invention, there is provided a polishing apparatus for polishing an object having a copper film on the surface to be polished, comprising a polishing tool having a polishing surface and having conductivity, a polishing tool rotating and holding means for rotating the polishing tool about a predetermined axis of rotation and holding the same, a rotating and holding means for holding a polishing object and rotating the same about a predetermined axis of rotation, a moving and positioning means for moving and positioning the polishing tool to a target position in a direction facing the polishing object, a relative moving means for making the polished surface of the polishing object and the polishing surface of the polishing tool relatively move along a predetermined plane, an electrolytic solution feeding means for feeding an electrolytic solution including a chelating agent onto the polished surface, and a current supplying means for supplying an electrolytic current flowing through the polishing tool through the electrolytic solution from the polished surface by using the polished surface as an anode and the polishing tool as a cathode.
According to the polishing apparatus of the present invention, for example, if a copper film with unevenness is formed on a polished surface of the object to be polished, the polished surface of the copper film is oxidized by the anodic oxidation by the current supplying means. This oxidized copper is chelated by a chelating agent in an electrolytic solution fed by the electrolytic solution feeding means. A chelate film of a rather low mechanical strength and able to be easily removed is thus formed.
The moving and positioning means brings the polishing surface into contact or proximity with the polished surface. The polishing tool rotating and holding means rotates the polishing surface and the polished surface in a state in contact or proximity with each other. Therefore, the projecting portion of a chelate film is removed. Further, by the relative moving means, projecting portions of the chelate film on the entire polished surface are polished and removed, therefore the polished surface can be polished efficiently at a low polishing pressure.
In the polishing apparatus of the present invention, preferably the electrolytic current supplying means comprises an anode member arranged to be able to be brought into contact or proximity with the polished surface and supply current to the polished surface using the polished surface as an anode and a DC power supply supplying a predetermined DC power between the anode member means and the polishing tool.
By locally passing a current from the anode member to the copper film through the electrolytic solution, a stable current can be supplied.
In this case, a current is supplied to the copper film from the anode member via the electrolytic solution and further from the copper film to the cathode member through the electrolytic solution, so the copper film near the polishing tool serving as a cathode is oxidized and chelated.
In the polishing apparatus according to the present invention, preferably the DC power supply outputs a pulse-like voltage of a predetermined period.
For example, setting the pulse width extremely short is effective for making the amount of the chelate film formed by the anodic oxidation per pulse very small, preventing sudden, huge anode oxidation of the copper film due to discharge due to a sudden change of the distance between electrodes in a case of contact with unevenness of the surface or the like, spark discharge due to a sudden change of electrical resistance occurring when air bubbles, particles, or the like are interposed, etc. and achieving continuity of amounts as small as possible.
In the polishing apparatus according to the present invention, preferably the anode member comprises a metal more precious than copper formed on the polished surface. Due to this, elution of the anode member to the electrolytic solution can be prevented, and the copper film can be actively oxidized by the anodic oxidation. Note since the cathode is essentially not eluted, it is not necessary to consider the preciousness of the cathode.
The polishing apparatus according to the present invention preferably further comprises a current detecting means for detecting a value of a current flowing from the polished surface to the polishing tool, more preferably further comprises a control means for controlling a position of the polishing tool in a direction substantially perpendicular with the polished surface so that the value of the current becomes constant on the basis of a detection signal of the current detecting means.
By control to make the current value constant, the current density becomes constant constantly and thereby the amount of the chelate film formed by the anode oxidation can be controlled constant.
In addition, to achieve the above object, according to a fifth aspect of the present invention, there is provided a polishing apparatus which comprises a polishing tool having a polishing surface in contact with the entire surface of the polished surface of the polishing object while rotating it and which brings the polishing object into contact with the polished surface while rotating it so as to flatten and polish the same, the polishing apparatus comprising an electrolytic solution feeding means for feeding an electrolytic solution including a chelating agent onto the polishing surface and an anode electrode and a cathode electrode capable of supplying electric power in the polishing surface and flattening the polished surface by electrolytic composite polishing which combines electrolytic polishing by the electrolytic solution and mechanical polishing by the polishing surface.
According to the polishing apparatus of the present invention, for example, by applying a voltage to the anode and cathode provided on the polishing surface, a current is supplied to the copper film on the polished surface from the anode member on the polishing surface via the electrolytic solution and furthermore from the copper film to the cathode on the polishing surface through the electrolytic solution, so the copper film of the polished surface near the cathode is oxidized.
This oxidized copper is chelated by the chelating agent in the electrolytic solution fed by the electrolytic solution feeding means, whereby a chelate film of a rather low mechanical strength able to be easily removed is formed.
By rotating the polishing surface and the polished surface respectively in a state contacting each other entirely or brought close to each other entirely, projecting portions of the chelate film on the entire polished surface are polished and removed, therefore the polished surface can be polished efficiently with a low polishing pressure.
In addition, to achieve the above object, according to a sixth aspect of the present invention, there is provided a polishing apparatus for polishing an object having a copper film on the surface to be polished, comprising a holding means for holding the object to be polished, an electrode plate arranged parallel with the polished surface, a vibration applying means for applying vibration on the polished object, an electrolytic solution feeding means for feeding an electrolytic solution including a chelating agent between the polished surface and the electrode plate, and an electrolytic current supplying means for supplying an electrolytic current flowing through the electrolytic solution from the polished surface to the electrode plate by using the polished surface as an anode and the electrode as a cathode.
According to the polishing apparatus of the present invention, for example, if a copper film with unevenness is formed on the polished surface, the polished surface of the copper film is oxidized by the anodic oxidation by the current supplying means. This oxidized copper is chelated by the chelating agent in the electrolytic solution fed by the electrolytic solution feeding means, whereby a chelate film of a rather low mechanical strength able to be easily removed is formed.
The projecting portions of the chelate film are selectively removed by the vibration action on the polished object by the vibration applying means. This enables efficient polishing causing little damage to the polished object.
The polishing apparatus according to the present invention preferably further comprises a current detecting means for detecting a value of a current flowing from the polished surface to the polishing tool. Thereby the electrolytic current can be monitored and the polishing process controlled, so it becomes possible to correctly grasp the state of progress of the polishing process.
In addition, to achieve the above object, according to a seventh aspect of the present invention, there is provided a polishing apparatus for polishing an object having a copper film on the surface to be polished, comprising a holding means for holding the polished object, an electrode plate arranged parallel with the polished surface, an electrolytic solution feeding means for feeding an electrolytic solution including a chelating agent between the polished surface and the electrode plate, an electrolytic current supplying means for supplying an electrolytic current flowing through the electrolytic solution from the polished surface to the electrode plate by using the polished surface as an anode and the electrode as a cathode, and a flushing means for flushing the electrolytic solution between the polished surface and the electrode plate.
According to the polishing apparatus of the present invention, for example, if a copper film with unevenness is formed on the polished surface of the object to be polished, the polished surface of the copper film is oxidized by the anodic oxidation by the current supplying means. This oxidized copper at an anode is chelated by the chelating agent in the electrolytic solution fed by the electrolytic solution feeding means, whereby a chelate film of a rather low mechanical strength able to be easily removed is formed.
The projecting portions of the chelate film are selectively removed by a vibration action on the polished object by the vibration applying means. This enables efficient polishing causing little damage to the polished object.
The polishing apparatus according to the present invention preferably further comprises a current detecting means for detecting a value of a current flowing from the polished surface to the polishing tool. Therefore, the electrolytic current can be monitored and the polishing process controlled, so it becomes possible to correctly grasp the state of progress of the polishing process.
In addition, to achieve the above object, according to an eighth aspect of the present invention, there is provided a polishing apparatus for polishing an object having a metal film on the surface to be polished, comprising a holding means for holding the polished object, a wiper for wiping the surface of the polished object, an electrolytic solution feeding means for feeding an electrolytic solution on the surface of the polished object, a facing electrode arranged at a position facing the surface of the polished object, and a current supplying means for supplying a current between the surface of the polished object and the facing electrode.
In addition, to achieve the above object, according to a ninth aspect of the present invention, there is provided a polishing apparatus for polishing an object having a metal film on the surface to be polished, comprising a holding means for holding the polished object, a wiper for wiping the surface of the polished object, a relative moving means for relatively moving the surface of the polishing object and the wiper, an electrolytic solution feeding means for feeding an electrolytic solution on the surface of the polished object, a facing electrode arranged at a position facing the surface of the polished object, and a current supplying means for supplying a current between the surface of the polished object and the facing electrode.
The relative moving means presses the wiper on the surface of the polished object and rotates the wiper relative to a predetermined center axis of rotation.
Alternatively, the relative moving means presses the wiper against the surface of the polished object and horizontally moves the wiper in the surface of the polished object.
Alternatively, the relative moving means rotates the holding means relative to a predetermined center axis of rotation.
Alternatively, the relative moving means horizontally moves the holding means in a surface parallel with the surface of the wiper.
According to the above polishing apparatuses according to the present invention, for example, when a metal film is formed on the polished surface of the polishing object, an electrolytic solution is fed onto the surface of the polishing object by the electrolytic solution feeding means, and a current is supplied between the surface of the polishing object and the facing electrode by the current supplying means, so the metal film is oxidized by the anodic oxidation, ionized into a state of metallic ions, and has a very low mechanical strength enabling it to be easily removed.
Further, by wiping the surface of the oxidized metal film using a wiper, the oxidized metal is removed, therefore, the polishing object can be polished efficiently even at a low pressure.